So I installed a new brake master cylinder on my 1990 civic hatchback (1.5 D15B) and after the install and bleeding, the brakes feel SUPER spongy, and have virtually no stopping power. I did bench bleed the master cylinder before installing it. My brake booster, brake pads, and calipers all less than a year old. Any advice would be appreciated.

 

Just need to bleed all 4 brakes now. Go in order - LF, RR, RF, LR. Did you measure and compare old vs new master cylinders depth from mounting face to the plunger inside the master cylinder? Sometimes the push rod from brake pedal through brake booster needs to be adjusted to compensate the difference.
Most likely you just have air in the lines still..